This invention relates to vibrating screen decks used for sizing pellets or other forms of material, and particularly to a replaceable fastener for holding the screen deck in contact with the vibration apparatus.
As the nation's supply of rich iron ore has been depleted it has been necessary to utilize taconite, a flint-like rock having a low iron content. The ore is crushed to a powder and the portions which are extremely rich in iron are magnetically separated from the remainder. The iron rich powder is mixed with small balls of bentonite in a ball mill where it coats the bentonite and forms pellets which form a very satisfactory charge material for steel making furnaces. For best results, the pellets must be of a uniform size. Sizing is accomplished by discharging the pellets from the ball mill to a vibrating screen deck which permits the undersize pellets to fall through while the larger pellets are transported to a second screen which separates pellets of the desired size from the oversized pellets.
The taconite pellets are handled in tremendous volumes. Since they are quite abrasive, it is usually necessary to replace the screen decks every two to four months. Conventionally, the large screen decks, which might have an overall size of about 2.5.times.6 m, comprise heavy guage stainless steel bars welded to underlying support rods. The deck is normally fastened, in sections of about 0.5 m width, to a series of spaced mounting bars carried by the vibrator apparatus. Typically, fastening has been accomplished by a large number of heavy bolts and nuts which must be assembled and removed by workmen operating in the very dirty and cramped space under the mounting bars. Many times, some of the holes in the new decks do not line up with the corresponding holes in the mounting bars, causing a great deal of difficulty in assembly. Also, the old bolts are usually so rusted and worn that they must be chiseled off.
With downtime for screen replacement currently costing about $600-$800 per hour and with replacement normally taking several hours it will be readily evident that a faster means to attach screen decks would be most desirable. Many screen deck users have attempted to eliminate the undesirable aspects of bolt type fasteners by utilizing C-shaped retaining clips formed of spring wire. These clips can be attached and removed from the top of the deck with relatively little effort. However, the fact that 300-400 clips or more are used on a given screen deck still requires considerable installation time. Unfortunately, it has been found that the C-clips that have been used in many installations have not been satisfactory. The clips are necessarily stretched as they are snapped over the mounting bar during installation, causing them to take a permanent set which considerably reduces their ability to resist the natural tendency of the screen deck to be separated from the mounting bars during the operation of the vibrator. Consequently, some of the C-clips are often stretched sufficiently to fall off the mounting bars shortly after the deck is first placed in operation. This places a greater load on the other clips and soon the screen deck may begin violently pounding against the mounting bars until it is damaged or the apparatus is shut down so that the lost and overly stretched clips can be replaced. When the apparatus must be shut down for clip replacement before the screen deck is worn out, the downtime savings produced by the use of clips rather than bolts and nuts is reduced or eliminated.
Obviously, it would be of tremendous benefit to screen deck users to have a fastening means that provides the long term fastening security of a bolt system and the assembly and disassembly convenience of the standard C-clip retainer.
It is among the objects of the present invention to provide an improved retaining clip which is as simple to install and remove as the conventional C-shaped clip currently being used but which takes a lesser set during assembly while providing greater holding forces as the screen deck tends to move further and further away from the mounting bars during vibration.